1. Field of the Invention
The present invention relates to microwave waveguide switching which is used in redundancy switching. Redundancy switching is used to enhance the reliability of communications satellites by switching in redundant system elements for those which have failed. It is common in microwave communications systems to provide stand-by units, such as receivers and transmitters, which are appropriately switched into the system to replace main units which have failed. With the improvement in network topologies, there has developed a requirement for complex microwave switching elements, such as a 4-port, 3-position "T" switch.
The switching apparatus needed in such redundancy switching systems was easy to provide when operation was at low frequency or at low transmission power levels. However, with high frequencies and high power levels, the existing switching apparatus has been inadequate. The deficiency is particularly acute in communication satellite applications, where the high per pound costs of an orbited satellite requires maximum possible switchable combinations of system elements. Further, the switch itself must be small and light in weight.
2. Description of the Prior Art
As described in pending application U.S. Ser. No. 670,290, filed Mar. 25, 1976, to Assal, et al., now Pat. No. 4,070,637, a switch needed for higher order satellite system redundancy configuration is one having four ports and three commuting states or positions. This switch is referred to as a "T" switch, and is illustrated schematically in FIG. 1 of the present application where the three positions are shown.
Since satellite communication systems operate at very high frequencies, the "T" switch must be able to operate efficiently at these frequencies. As is well known in the microwave art, waveguide switches have the best electrical characteristics at these high frequencies. However, the known waveguide switches which provide the three connecting states or positions required in the "T" switch are not suitable in satellite communication systems because of several deficiencies which include excessive size, weight and cost.
A known non-waveguide switch which provides the three connecting states of the present invention is the microwave matrix switch of Lee Laboratories, Lexington, Massachusetts. This microwave matrix switch uses connectors instead of waveguide ports and, therefore, is unsuitable for satellite communication systems where low loss and high power capability are required.
A known waveguide switch called an "R" switch is illustrated in FIG. 2. This switch can provide only the first and second connecting states of the "T" switch which are indicated in FIG. 1. One such "R" switch is available from Sivers Labs in Stockholm, Sweden, and is designated PM 7306J. As shown in FIG. 2, the "R" switch can provide the first connecting state with port 1 connected to port 2 and port 3 connected to port 4. The "R" switch can also provide a second connecting state with port 1 connected to port 3 and port 2 connected to port 4. The "R" switch, however, cannot provide the third connecting state of the "T" switch since port 1 cannot be connected to port 4 simultaneously with port 2 being connected to port 3. This is because the "R" switch has no means to accomplish the cross-over which is required for the third state.
A known waveguide switch called the modified "R" switch does provide the three connecting states of the present invention. The modified "R" switch accomplishes the third state by providing a fourth connecting path which passes beneath the other connecting states in a manner such that the rotor has ports at two levels. As is apparent below, however, the modified "R" switch has several major deficiencies with respect to satellite communications systems, including excessive size, weight and cost.
The modified "R" switch, as shown in FIGS. 3 and 4, includes an unmodified "R" switch. FIGS. 3 and 4 show the modified "R" switch, and the unmodified portion will hereinafter be described first. The "R"[switch is housed in a square structure designated generally by reference numeral 30. Ports 20, 22, 24 and 26 are provided in successive 90.degree. angles around structure 30. For purposes of description and with reference to FIGS. 2 and 4, port 20 corresponds to port 1, port 22 corresponds to port 2, port 24 corresponds to port 4, and port 26 corresponds to port 3. A structure 28, mounted for rotation on drive shaft 10, is provided in structure 30. A waveguide 12 is mounted to structure 28 and has a length such that it can electrically couple port 22 to port 26, or port 20 to port 24, depending on the angle of rotation of the shaft 10. A curved waveguide 16 is mounted to structure 28 and has a curve and a length such that it can electrically couple port 20 to port 22, port 22 to port 24, port 24 to port 26, or port 26 to port 20, depending on the angle of rotation of shaft 10. Waveguide 14 is similarly mounted on structure 28 and connects port 24 to port 26, port 26 to port 20, port 20 to port 22, or port 22 to port 24, depending upon the angle of shaft 10. Obviously, with this unmodified "R" switch, it is impossible to provide the third connecting state of a "T" switch because port 20 cannot be connected to port 24 simultaneously with the connection of port 22 to port 26.
In order to provide the third connecting state required of a "T" switch, the "R" switch can be modified in the following fashion. Specifically, as shown in FIG. 3, a waveguide 18 having four 90.degree. bends can be mounted in structure 28 perpendicular to and below waveguide 12. Waveguide 18 has a length such that it can electrically couple port 20 to port 24 or port 22 to port 26, depending upon the angle of rotation of shaft 10. It is noted from FIGS. 3 and 4 that waveguide 18 accomplishes this cross-over by lying in a plane beneath that of waveguides 12, 14 and 16.
While the modified "R" switch provides the three connecting states or positions required for a "T" configuration, it has several major deficiencies. First, in order to provide waveguide 18, the height of structure 30 has to be at least doubled, and the length and width of structure 30 has to be increased to accommodate the four required 90.degree. bends in waveguide 18. In satellite applications, weight is extremely critical, and this increase in size, and hence weight, is a serious deficiency. Secondly, the addition of waveguide 18 requires a larger diameter shaft and a larger source to drive shaft 10.
Yet another solution to the problem of providing the third connecting state with a cross-over capability is found in copending Application Ser. No. 851,659 to Berman, et al. In that application, there is shown the waveguide switch as shown in FIG. 6 of this application. This waveguide switch is dependent upon six resonant cavities designated as 40, 42, 44, 46, 48 and 50. By tuning and detuning these resonant cavities, the necessary connections to provide the three switching states of the "T" switch are provided. As shown in the electrical analogy of FIG. 5, the cross-over is accomplished by the crossing of the paths of resonant cavities 48 and 50 at different levels.